They are currently stored in the file ~/.config/gmusicbrowser/tags (gmbrc in v1.1.x) with all the rest. I will probably save them in individual m3u files in the future.

Currently, the only way to back them up, is to use the export plugin to add the "export to m3u" option in the context menu, and for each list display it, select the songs, export them. I'll add a more direct export option.

And to import them, I only added this function recently in v1.1.x (wait for 1.1.4 or use a git snapshot)